There are numerous situations where it is desired to provide intelligible sound reproduction in environments having highly variable ambient noise levels. Such situations are routinely encountered in the use of public address systems, background music systems, paging systems, and both civilian and military mobile communication systems.
The most straightforward and frequently employed approach to compensating for variable ambient noise is to provide the sound reproduction system with a variable gain or volume control which is responsive to the ambient noise level. In such systems, the volume of the sound reproduction system is varied in dependence on the noise level so as to maintain a signal-to-noise ratio which is sufficiently great to ensure that the data being reproduced by the sound system is discernible or intelligible over the presently existing background noise. Examples of such prior art volume control systems are disclosed in the following U.S. Pat. Nos. 2,338,551 (Stanko); 2,420,933 (Crawford et al); 2,486,480 (Kimball et al); 2,616,971 (Kannenberg); and 3,290,442 (Suganuma). It is to be noted that the foregoing list of patents is not, and is not intended to be, exhaustive of the prior art.
The basic approach employed in each of these systems is simply to obtain a control signal by subtracting a signal corresponding to the desired data signal to be reproduced from a signal corresponding to the reproduced audio data plus the ambient noise present at the time of reproduction. More specifically, the Stanko system compares a portion of the input signal with the filtered output of a microphone which senses both the loudspeaker output and the ambient noise. The microphone output is inverted in polarity so that it opposes the input signal and the data portion of the microphone output is thus cancelled from the compared signal. The compared signal is rectified and the rectified signal used to control a variable gain amplifier.
In the Crawford et al system, the output of a microphone which senses both the loudspeaker output and ambient noise and a compensating voltage corresponding to the program signal are applied to the control grid of the amplifier tube. As the ambient noise level changes, the amplification factor of the amplifier tube changes correspondingly.
In the Kimball et al system, opposing currents representing the aggregate intensity of all sounds detected at a predetermined control point and just the speaker output, respectively, are applied to a resistance network such that the total voltage drop thereacross is proportional to the difference between the two currents. This voltage is used to control electric motors which mechanically actuate volume-control potentiometers.
The Kannenberg system is similar to the Stanko system, in that the gain of a variable gain amplifier is varied in accordance with the difference of two rectified voltages, but the Kannenberg system also contains a limiting circuit.
In the Suganama system, both the program signal and the combined noise and program signal are rectified to eliminate waveform and phase differences between the two signals, and the two rectified signals are combined to actuate driving mechanisms for control potentiometers in a volume control feedback network.
However, such systems suffer from a number of disadvantages. Those systems which utilize filtering, such as the Stanko system, cannot be used in situations, such as those encountered while mobile communication systems, where the noise is random in nature. Those systems which utilize mechanically driven potentiometers, such as the Kimball et al and Suganuma systems, have response times which are too slow for use in other than a relatively stable environment such as an auditorium, and are completely unsuited for use in mobile communication systems and the like. Further, all of the systems have only a limited dynamic range and must provide some form of limiting in order to avoid saturation caused by noise or program induced voltage runaway.